The present invention generally relates to forming of images on a sheet and more particularly to a method of forming an image on both sides of a sheet.
When forming a number of images on both sides of a number of sheets as in a photocopier, printer, facsimile and the like, a method is conventionally adopted in which the formation of the images on one side of the sheet is first completed for the entire sheets and the sheets thus formed with the images are stored once in a suitable means such as a tray. Then, the formation of the image is performed on the other side of the sheets by feeding the sheets from the tray one by one after turning over of the sheets by a sheet turn-over mechanism and the like.
For example, when taking a copy of a sixty page document printed on both sides of a thirty sheet of papers, thirty images on a first side of the original sheets are consecutively recorded on thirty copy sheets by feeding the copy sheets one by one and these thirty copy sheets thus recorded with the images are stored in an intermediate tray. After the copy of the thirty images is completed, the stacked copy sheets are fed one by one again and the images on a second side of the original sheets of the document are recorded on the other side of the copy sheets consecutively. Such a method of image formation is called a "stack mode".
In such a prior art image formation method, it is necessary to separate the sheets on the intermediate tray one by one with absolute reliability. When the separation of the sheets is defective and two or more sheets are fed at the same time or feeding the the sheet is jammed, there occurs a problem that the reproduction of the entire document is failed.
Further, as it is usual to set the sheets on the intermediate tray for proper feeding by laterally pressing both sides of the sheets stacked on the tray by a pair of guide plates, there is a problem in such a prior art method in that sheets having different sizes cannot be used or mixed. In other words, copy of documents containing sheets or papers having different sizes cannot be made according to this method.
Further, in order to handle a case in which a jam or other failure has occured in the feeding of the sheets, an information indicating the page of the document at which the failure has occured is desired to be produced. Otherwise, the entire document has to be read from the beginning. This means, however, that the information about the feeding of the sheet has to be stored in a controller for the entire sheets and the load of controlling task of the controller is increased.